I have a problem which is basically, driving me nuts!

I have a '83, 8 valve, non ABS K100. Instruments were replaced with a later year unit from the UK (MPH) but my 'problem' precedes this fix. Also, the brakes on the bike are in a very good condition. New brake pads front and rear. New brake disk rear (font within 90% of new spec.); new fluids throughout and new master cylinder kit on the rear. Basically, the following:

When I start the bike, the brake warning light comes on. A quick touch to front and back brake levers shuts the warning light off and it stays off. When I switch on the park lights, the brake warning light stays off. When I switch on the main beam, the light comes on and stays on while the main beam is on.

If I delay switching on the main beam for approx 500m of travel; and then switch on, the warning light stays off - sometimes. Or, when it is off and I apply hard brakes; either front or rear, it will come on - or not. 

This little light seems to have a mind of its own. I've looked at the wiring diagrams and the books, but cannot comprehend how the brake warning light and the main beam may be 'connected'.

Connections are clean and sealed. The instrument cluster has been opened and cleaned - and sealed again. This problem persisted before I changed the instruments (bad speedo gears) and is still here - despite my best efforts.

Now I need your expert help.

The rest of the bike? Magnificent!

Exact same problem developing here with my 85 RT. I think mine has something to do with the parking light globe/fitting as it will occur if I start the bike with parker on , but not the main beam. Sometimes. I think my whole headlight assembly is a bit loose and something is moving. Carry a piece of black electric tape to put over the light. (until solved anyway).
Good luck.

What I see in common there is the bulb monitoring unit and that the problem is there when the park is on and goes when the main light comes on makes me even more inclined to check the BMU. Yours being an 83 and RTs is an 85 so both have reed relays. I would be inclined to change it for a later type with solid state and go from there.
No guarantees but worth a try.

Thank you Obe Wan, I shall do just that.